1. Field
A premixing burner and a method for operating a combustion chamber by means of a liquid and/or gaseous fuel are disclosed, with a swirl generator for a combustion inflow air stream for forming a swirl and with means for the injection of fuel into the swirl flow, the swirl generator being adjacent to the combustion chamber indirectly via a mixing zone or directly, in each case via a burner outlet, a cross-sectional widening at the burner outlet being provided, which is discontinuous in the flow direction of the swirl flow and through which the swirl flow bursts open so as to form a backflow zone.
2. Background Information
Premixing burners are known from a multiplicity of prior publications, such as, for example, EP A1 0 210 462 and EP B1 0 321 809, to name only a few. Premixing burners of this type are based on the general operative principle of generating, within a swirl generator mostly designed conically and providing at least two part-conical shells assembled with a correspondingly mutual overlap, a swirl flow which consists of a fuel/air mixture and which is ignited within a combustion chamber, following the premixing burner in the flow direction, so as to form a premixing flame which is then as stable as possible in spatial terms. In this case, the spatial position of the premixing flame is determined by the aerodynamic behavior of the swirl flow, the swirl coefficient of which increases within an increasing propagation along the burner axis and which consequently becomes unstable and ultimately, due to a discontinuous cross-sectional transition between the burner and combustion chamber, bursts open into an annular swirl flow, so as to form a backflow zone, in the front region of which the premixing flame is formed.
The vortex backflow zone has only limited stability properties, and therefore there have already been a multiplicity of proposals for improving the stability properties of backflow zones of this kind. For as stable a vortex backflow zone as possible, it is essential that the axial profile of the swirl flow generated by the swirl body should have a low swirl in the center, that is to say the region of the burner axis, and, moreover, an axial velocity excess should be present there. These considerations have led to a burner according to EP 0 321 809 B1.
The double cone burner described in this publication is shown in FIG. 2 diagrammatically in the form of a longitudinal sectional illustration and has a conically designed swirl flow generator 1, of which the two part-conical shells placed one in the other in each case enclose two air inlet slits 2. The swirl generator 1 issues at the burner outlet 3 directly into the combustion chamber 4 via a discontinuous cross-sectional widening. By the combustion inflow air being fed in tangentially along the air inlet slits 2, a swirl flow is generated which is propagated in the axial flow direction with an increasing swirl about the axial direction of the swirl generator. On account of the increasing swirl in the axial flow direction, the instability of the swirl flow increases and merges into an annular swirl flow with backflow. A backflow zone 5 is formed essentially within the combustion chamber 4 in the region of the burner outlet 3, with a front located forward in the flow direction or with a forward stagnation point 6, of which the axial position in relation to the premixing burner 1 is to be determined essentially by the cone angle 2γ and the slit width of the air inlet slits 2. The size and appearance of the backflow zone 5 can be essentially determined by the choice of size of the above geometric values.
Within the backflow zone 5, the premixing flame 7 is formed, which is stabilized at the front region of the inner backflow zone 5.
Investigations into the stability of a flame 7 of this kind have shown that the aerodynamic stability of the inner recirculation or backflow zone 5 has a decisive influence on the position, shape and size of the premixing flame 7.
If the above-described premixing burner serves for the generation of hot gases for driving a gas turbine plant, then, for reasons of the optimization of the efficiency of the gas turbine plant, it is appropriate to keep the pressure loss across the burner as low as possible. Since the swirl coefficient and pressure loss are in direct proportion to one another, it is desirable to have as low a swirl coefficient as possible within the swirl flow, which should be selected so as to be just high enough to ensure that an inner backflow zone is formed.
On the other hand, the aim must be to keep the forward stagnation point 6 of the backflow zone 5 as stable as possible aerodynamically, in order to prevent the situation where, due to a pronounced variation in the flame position, the premixing flame front anchored at the forward stagnation point 6 causes thermoacoustic instabilities which not only have a persistent influence on the efficiency of a gas turbine plant, but, moreover, cause considerable material stresses on almost all the components in the gas turbine plant which are in direct contact with the hot gases, with the result that the overall lifetime of the plant is ultimately reduced. However, the desire for as high an aerodynamic stability as possible in the forward flame front within the backflow zone is in contradiction with the efficiency-induced reduction in the swirl coefficient which leads to lower swirl gradients in the burner, in particular at the location of the forward stagnation point 6. However, a lower swirl gradient implies a greater deflection at the stagnation point in the flow direction, possibly with disturbances occurring, and is conducive to the above-described formation of thermoacoustic instabilities.